Good post. The words leave/stop/unjoin/cancel are all correct to use but wait!
When deciding on some names or words to use for a particular button especially when it comes to an interactive environment, you have to be very careful in choosing what will not psychologically put off a new or an engaged user.
"Unjoin this app". The meaning, at first thought refers me to inanimate objects like "unjoining" a paired or tied wood or material. We humans see an app as a "virtual" friend that we use in the absence of our "real" friends. so though the name unjoin is correct but never seems so to a natural human. So pls dont tell me you want to unjoin me.
"Stop this app". Sounds like something bad has happened and needs immediate disconnection.
Like an unforeseen error occurred and needs to be stopped. Just needs to stop becomes I now hate you. Why continue with you when you have hurt me, all i need is to pls "Stop me now"
"Cancel this app" Seems like the second best option but i will personally not use it in this scenario because am thinking that after I cancel you, there is no option of bringing you back. Take for instance when you taking a multichoice exam with a biro. A done mistake cannot be erased. Rather a fresh sheet of paper is needed or your work will look untidy.
"Leave this app". Sounds very temporal. Meaning that I am not yet through with you but will be back another time. Very modest and soothing to a human being. Its response is never harsh and came without any harm. So I leave you for now and go and do other things but will back someday for another ride.
So I give it up for Leave this app or any text with "leave" as the best option to use in this scenario and likewise.